1. Field of the Invention
This invention relates to apparatus and method for fractionation, for example, for recovering gasoline and light cycle oil from bottoms product of a main column associated with a fluid catalytic cracking system.
2. Description of the Prior Art
In fluid catalytic cracking operations, because gasoline and light cycle oil (LCO) are economically more valuable than main column bottoms (MCB) product, it is desirable to recover light ends from MCB product. The light ends content of MCB product depends on the operating conditions of the fluid catalytic cracking (FCC) main column fractionator unit, and particularly, on the flash zone temperature which is limited to a maximum value because of increased coking tendency of heavy hydrocarbons at elevated temperatures. The maximum flash zone temperature requirement limits the separation obtainable between LCO and MCB product. Typically, approximately 10% of the MCB product comprises LCO and lighter components.
FIG. 1 illustrates a conventional system using a single side stripper 67 associated with main column fractionator 115. MCB product is withdrawn along line 101 as residuals product. Side draw 123 from main column 115 is passed to stripper 67, with overhead product from stripper 67 being recycled to main column 115. Stripping steam is introduced via line 127 into stripper 67 and LCO is withdrawn from stripper 67 along line 119. Also, receiver/separator 117 receives main column 115 overhead, after partial condensing, to provide recovered products via lines 49 and 53. This system is disadvantageous in that the MCB product contains a significant quantity of light components.